1. Field of the Invention
The invention relates to fluorescent lamps which contain mercury, and is directed more particularly to the means by which mercury is introduced into such lamps.
2. Description of the Prior Art
Fluorescent lamps require a minimum quantity of mercury to reach rated life. If an insufficient amount of mercury is disposed in the lamp, the lamp will not provide full light output for its rated life. In fabrication of fluorescent lamps, a current method for dispensing (or xe2x80x9cdosingxe2x80x9d) mercury into the lamp includes mechanically dispensing the mercury into the lamp via a passageway, known as an exhaust tube. This has several disadvantages. First, control of the quantity of mercury dispensed is accomplished by mechanical means which slice off a body of mercury which falls into the lamp via the exhaust tube. If the mechanical slicer fails to operate properly, some lamps can contain no mercury while other lamps receive double dosing. This technique has other disadvantages. Safety issues associated with handling mercury in the lamp fabrication environment require special equipment, including respirators. If the dispenser becomes inoperative, the exhaust tube must be shut down and the production off that unit interrupted until the entire assembly is replaced. The dispensing system also requires periodic cleaning and rebuilding since it is a mechanical system.
Other dosing systems include use of a glass mercury ampule surrounded by a ferro magnetic strip. The entire ampule assembly is attached to a fluorescent mount by a spud wire. The ampule consists of a glass envelope with a fuse wire which provides continuity with the ferro-magnetic strip. The entire mount is sealed into the lamp and the mercury is released by means of R.F. energy. The lamp is exposed to an R.F. source which couples with the ferro-magnetic strip. The current in this loop heats the fuse wire which melts the glass and breaks the hermetic seal, releasing the mercury into the lamp. This system has many disadvantages. It requires a complex cathode with extra components, including the ampule, fuse wire, ferro-magnetic strip or shield, and spud wire. These extra components increase the material cost of the lamp and add processing steps which result in greater expense.
Other systems to dose mercury include provision of strips with a mercury compound embedded on the surface of the strip. When the mercury compound is heated to a sufficient temperature, mercury is released from the surface. The means to heat the material to an activation temperature employ R.F. energy to couple with the strip. The material must be heated to high temperature, of a magnitude of 800xc2x0 C., for a period of 10 to 15 seconds. This results in many problems since the other components on the finished lamp cathode cannot tolerate heating to a high temperature. These components include the two leadwire filament supports, the tungsten filament itself, and the emissive coating material, which is in an oxide form. If any of these components are heated to a high temperature, they can outgas and contaminate the lamp, which results in poor lamp performance. Since the strip material must be heated to a high temperature for a long duration, it is difficult to perform activation without damaging the other components. Another disadvantage of the strips with mercury compounds is that the strip must have a large surface area to contain enough mercury to sustain the lamp for its rated life. This results in unduly large cathode structures, which are difficult to process on lamp making equipment. The final drawback to this approach is that there is no means to determine if the strip is heated to a sufficient temperature and maintained at that temperature sufficiently to dispense or release all the mercury from the surface of the strip. If this is not the case, the mercury which was not released remains on the surface of the strip in an unusable form. This can result in failure to meet rated lamp life. Again, the strip must be mounted by a spud wire which adds to the complexity of the design. The strip system adds incremental cost which is undesirable.
Another method to dispense mercury consists of using a deep drawn tube of a suitable steel alloy, depositing the mercury into the tube with a dispensing mechanism, or injector, and crimping it together with a cold fusion weld to hermetically seal it. The capsule can be attached to the clamp area of the inner leadwire and processed on an exhaust machine. After the exhaust process, the capsule can be opened by any suitable means, including anode heating via collecting rectified current through the capsule, which causes the capsule seal to fail and the mercury to release. R.F. heating can also be employed. This system also has disadvantages. The dimensions on the cold weld seal are critical to proper operation and difficult to maintain. If the seal is too weak, the mercury may leak out early in the exhaust process and result in a low or no mercury lamp. The shape of the capsule also poses many problems. Since the part is not symmetrical, it is difficult to feed and handle on production equipment, which results in unneeded downtime and scrap. The capsule itself is also expensive to fabricate. The capsule is attached to the leadwire by welding which is difficult and can potentially damage the cathode if any material is sputtered during the welding process.
Accordingly, there is a need for an improved mercury capsule for use in fluorescent lamps, which capsule is of low cost, easy to handle, facilitates a mercury-free lamp production environment, facilitates easy measurement of precise doses, and is inexpensive to make and simple and inexpensive to use in lamp fabrication.
With the above and other objects in view, as will hereinafter appear, a mercury capsule for use in a fluorescent lamp comprises a metal ribbon which, in turn, comprises a first portion having a depression formed in a surface thereof for receiving and containing mercury, a second portion having a protrusion formed on a surface thereof, and a bendable portion interconnecting the first and second portions. The second portion is bendably movable to a position wherein the protrusion overlies the depression and is further movable to place the protrusion in sealing engagement with the depression to sealingly enclose mercury in the depression.
In accordance with a further feature of the invention there is provided a mercury capsule for use in a fluorescent lamp, the capsule comprising a metal ribbon comprising a first portion having a depression formed in a surface thereof, and a second portion having a protrusion formed on a surface thereof. The first and second portion surfaces are adjacent each other and the protrusion is sealingly engaged in the depression to define a chamber in the depression. The first and second portions are interconnected by a bent portion facilitating the adjacency of the first and second portion surfaces. A selected quantity of liquid mercury is disposed in the chamber.
In accordance with a still another feature of the invention, there is provided a fluorescent lamp having an envelope of light-transmitting vitreous material, having opposed end portions, first and second electrodes respectively disposed within the opposed end portions, and a pair of lead-in wires connected to the first and second electrodes, and containing an inert starting gas. An improvement comprises a mercury capsule secured to one of the lead-in wires. The mercury capsule comprises a metal ribbon comprising a first portion having a depression formed in a surface thereof, and a second portion having a protrusion formed on a surface thereof. The first and second portion surfaces are adjacent each other and the protrusion is sealingly engaged in the depression to define a chamber in the depression. The first and second portions are interconnected by a bent portion facilitating the adjacency of the first and second portion surfaces. A selected quantity of liquid mercury is disposed in the chamber.
In accordance with a still further feature of the invention, there is provided a method for making a mercury capsule for use in a fluorescent lamp, the method comprising the steps of providing a metal ribbon comprising a first portion having a depression formed in a surface thereof, and a second portion having a protrusion formed on a surface thereof, and a bendable portion interconnecting the first and second portions. The method further includes the steps of placing a selected amount of liquid mercury in the depression, bending the bendable portion to move the second portion to a position wherein the protrusion overlies the depression, and further bending the bendable portion to move the protrusion into sealing engagement with the depression to sealingly enclose the mercury in the depression.
The above and other features of the invention, including various novel details of construction and combinations of parts and method steps, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular device and method steps embodying the invention are shown by way of illustration only and not as limitations of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.